This invention relates to stainless steel sheets useful as exterior members having improved workability and weatherability on uncoated surface thereof, and their processes for making the same. It also relates to pipe-making stainless steel sheets having improved corrosion resistance and workability at weld joints.
Exterior members as typified by exterior building panels and automotive exterior members such as bumpers and side moldings are often made of stainless steel because aesthetic appearance and weatherability are required. Useful stainless steels are SUS 434, SUS 304 and stainless steels having improved corrosion resistance due to Nb and Cu added in combination.
Steel material from which welding pipes are prepared must itself exhibit corrosion resistance, workability and weldability. Since welded pipes prepared therefrom are often subjected to severe working, the welded pipes themselves are required to have improved corrosion resistance and workability at weld joints. Typical of the material which is conventionally used to make welded pipes such as automobile exhaust gas conduits/pipes is aluminized steel comprising a cold rolled steel substrate having aluminum hot dipped at high temperatures (see Japanese Patent Application Kokai No. 60-152663).
Environment pollution becomes more serious in these years. The environment is now more corrosive as by acid rain particularly in Europe and North America. In addition, rock salt is often dispersed in winter on the road to prevent freezing. Because of these factors, the environment becomes more severe which exterior members like automotive bodies and exterior building members and welded pipes like automotive exhaust gas conduits must withstand. Even the above-mentioned stainless steel sheets used as exterior members suffer from the problem that their appearance is impaired by rust or stain formation. There is a need for the development of highly corrosion resistance stainless steel sheets having improved weatherability.
In general, for the purpose of improving the corrosion resistance of stainless steel, it is known to increase the content of chromium or further add molybdenum. Unfortunately, these approaches not only add to the cost of stainless steel, but result in reduced workability, rendering difficult press forming into a complicated shape.
Stainless steel sheets are used as automobile exterior members such as side moldings, body locker panels, wheel arch moldings, and bumpers. The body to which these exterior members are attached is electrochemically less noble or lower in electrochemical series than stainless steel. Thus the body undergoes galvanic corrosion and eventually cosmetic corrosion in that a lacquer coating is broken to adversely affect the aesthetic appearance.
One known corrosion resistant stainless steel sheet which can prevent the cosmetic corrosion of the associated body and is of light weight is a cold rolled aluminum clad stainless steel sheet as disclosed in Japanese Patent Publication No. 47-19853. The aluminum which is electrochemically less noble than ordinary steel intervenes between the body-forming ordlnary steel and exterior decorative stainless steel. The aluminum provides for sacrificial corrosion prevention, preventing the cosmetic corrosion of the body.
Such clad stainless steel has the problem that it cannot be press formed into a complicated shape because stainless steel is hardened during cladding of stainless steel with aluminum by cold rolling and cannot be softened by annealing in a temperature range below the melting temperature of aluminum (660.degree. C). Cladding under pressure contact by cold rolling tends to introduce flaws on the surface of stainless steel sheet to be used as an exterior surface. Buffing is thus necessary to eliminate such flaws so that the product becomes very expensive.
It is often desirable in view of productivity to mount exterior members to the automobile body at some feasible positions by spot welding. In preparing the above-mentioned cold rolled aluminum clad stainless steel, a higher cladding ratio of aluminum must be employed because of cold roll cladding conditions. The cladding material or aluminum thus has a substantial thickness. Since the melting point of aluminum is greatly different from that of the body-forming ordinary steel, it is impossible to spot weld alumimum to ordinary steel. This invites a substantial reduction in productivity of automobile manufacture.
The worsening corrosive environment mentioned above also imposes a problem on welded pipe-making material. The corrosion resistance of the above-mentioned aluminized steel is insufficient and the outside of a pipe at a weld joint is substantially corroded to such an extent that a pore might be formed across the pipe wall. The recent trend in the manufacture of exhaust gas conduits is to replace the aluminized steel by 11-13% Cr stainless steels having higher corrosion resistance than aluminized steel, such as SUH 409 and SUS 410. Although they are stainless steels, they are not satisfactorily corrosion resistant under the worsening corrosive environment because of their low chromium content (Cr 11-13%), and suffer from substantial corrosion particularly at weld joints.
As previously described, it is known to increase the content of chromium or further add molybdenum to stainless steel to improve the corrosion resistance thereof. A further increase of chromium content or addition of molybdenum to the above-mentioned stainless steel is undesirable as the welded pipe-making material because not only the cost is increased, but the workability of the material itself or the workability of the pipe at a weld joint is lowered. The welded pipe-making material is required to have weldability and workability in itself because it is shaped into a pipe by roll forming the material into a round shape and welding the mating edges by TIG or high frequency welding. Even after the material is shaped into a pipe, the pipe is further worked into a suitable shape. Thus the material must be so workable that no crack may occur in the material itself and at welded joints during subsequent working of the pipe.